Zirconium dioxide (ZrO.sub.2) is one of the more widely studied ceramic materials. It is known that zirconium dioxide can exist in a stable or metastable state as one or more of four phases: the monoclinic, orthorhombic, tetragonal, or cubic crystal structures. The phases present, their amount, size, and distribution can have a profound influence on a whole range of material properties in a composite with another ceramic. A significant level of interest in zirconia (ZrO.sub.2) -- based ceramics has surfaced in recent years in an attempt to understand and engineer the microstructure of these composites. An engineered microstructure can result in improvements in strength, thermal shock resistance, and fracture toughness values over those of the ceramic without the zirconia addition. In this case zirconia is an effective `toughening agent` because of a phase transformation in which a volume expansion can occur. Several toughening mechanisms have been theorized and all are process dependent. References by Lange.sup.1, Evans.sup.2, Claussen.sup.3 et al., and Evans & Heuer.sup.4, give full explanations of this phenomenon. Processing of these materials has focussed along three routes, all having the same result of obtaining a dispersion of ZrO.sub.2 particles in some ceramic matrix. FNT .sup.1 Lange, F. F., J. Material Science, 17 (1-4), 225-255, 1982. FNT .sup.2 Evans, A. G. and McMeeking, R. M., Mechanics of transformation toughening in brittle materials, J. Am. Ceram. Soc., 65 [5]242, 1982. FNT .sup.3 Claussen, N. and Ruhle, M., "Design of Transformation Toughened Ceramics", p. 137, Advances in Ceramics Vol. 3, Science and Technology of Zirconia, 1981. FNT .sup.4 Evans, A. G. and Heuer, A. H., "Transformation Toughening in Ceramics:Martensitic Transformations in Cracktip Stress Fields", J. Amer. Ceram. Soc., 63 [5-6]241-48, 1981.
A conventional approach involves mechanically blending or mixing two (or more) components or their precursor powders together, adding organic lubricants and binders, pressing into a shape, and sintering at a temperature high enough to promote densification. However, zirconium dioxide requires a relatively high sintering temperature (&gt;1700.degree. C.) to attain the densities required for these toughening mechanisms to operate. The acceptance of improved mixing technologies to ensure uniform homogeneity of the zirconia phase, along with technological improvements in pressing and sintering (e.g. isostatic pressing, hot pressing, hot isostatic pressing), have effectively lowered the sintering temperature necessary for densification to more practical levels (.about.1450.degree.-1700.degree. C.). A lower sintering temperature is advantageous because it also minimizes excessive grain growth of the zirconia particles which consequently improves metastable phase retention levels and strengths.
Tough zirconia ceramics have been fabricated from spinel (of fused cast origins), silicon carbide, and mullite. (All show improvements in fracture toughness, strengths, and in some cases, thermal shock resistance.) However, the most widely studied system is the aluminum oxide-zirconium oxide (Al.sub.2 O.sub.3 --ZrO.sub.2) system, where particles of stabilized or unstabilized zirconia are added to Al.sub.2 O.sub.3, mixed, and densified.
Another method for making toughened zirconia ceramics involves a conventional powder oxide (or precursor) mechanical mixing approach using zirconium oxide and a stabilizer. Conventional high temperature sintering is done with subsequent heat treatments at lower temperatures to produce a "composite" of two or more of the zirconia phases. The shape, size, and homogeneity of this phase (phases) is determined by the heat treatment employed which in turn, determines the final properties of the ceramic article. The amount of stabilizer added also influences the type and amount of minor phase precipitates formed. Conventional processes are used either in the powder preparation and/or the powder mixing step.
A third method of obtaining a dispersion of ZrO.sub.2 particles in a ceramic matrix involves a chemical reaction to produce these particles and matrix in-situ from completely different starting constituents. For example, a well documented reaction involves that of zircon and alumina to form zirconia in a mullite matrix as follows: EQU 2 ZrSiO.sub.4 +3 Al.sub.2 O.sub.3 .fwdarw.2 ZrO.sub.2 +3 Al.sub.2 O.sub.3 .multidot.SiO.sub.2
Again, this reaction involves the mixing of two already formed oxide materials, physically blending them to ensure homogeneity, and hot-pressing them to carry out the reaction at a lower temperature.
Some references cite examples where one or more of the zirconia composite constituents were chemically produced and physically mixed to form the composite powder. But, to the best of our knowledge, no information is available on processes where the constituents of the zirconia toughened ceramic have been simultaneously precipitated. One of the constituent materials can be the stabilizer precursor salt for the zirconia phase. One advantage to this process is the homogeneity achieved with the constituent precursors. This can eliminate the need to further mechanically blend the powders in subsequent processing. Another advantage lies with the chemical precipitation process itself. This process produces finely divided, pure precipitates which, when processed and dried properly, yield highly active, sinterable powders. This can alleviate the need for expensive pressing operations (i.e. hot-pressing) to achieve lower sintering temperatures for densification into a fine-grained ceramic unit.